Catalytic applications of rhenium compounds and hydrogen atom transfer reactions of substituted phthalimide N-oxyl radicals

摘要

In this work, methyltrioxorhenium (MTO) was found to be an active catalyst for two reactions: one is the reduction of hydronium ions by Euaq 2+ to evolve H2; the other is reduction of perchlorate ions to chloride ions by Euaq2+ or Craq 2+ in acidic solution. Kinetic studies were carried out and reaction mechanisms were proposed to agree with all the experimental data. In the hydrogen evolution reaction, a rhenium(V) hydride complex was postulated in the scheme to generate H2 by a proton-hydride reaction. Under similar conditions, Cr2+ ions do not evolve H2, despite E° Cr ~ E°Eu. In addition, no H2 formation were observed in the presence of perchlorate ions because the reaction between methyldioxorhenium (MDO) and perchlorate ions has a much faster rate than that of hydrogen evolution.;A six-coordinate rhenium(V) compound MeReO(edt)(bpym) was prepared, characterized, and investigated for oxygen atom transfer reactions between picoline N-oxide and triarylphosphines. We found it is a good catalyst for the reaction, even though it is less active than those five-coordinate rhenium(V) dithiolato compounds. The kinetics showed that the reaction has a first-order dependence on both rhenium and picoline N-oxide. Triarylphosphines were found to inhibit the reaction, and those phosphines with more electron-donating groups in para positions had slower reaction rates. This study proves a hypothesis: there should be a steric requirement for the potential catalyst in the oxygen transfer reactions, which is the necessary existence of an open coordination site on rhenium center.;In the last chapter, we studied three different types of reactions of phthalimide N-oxyl radicals (PINO·) and N-hydroxylphthalimide (NHPI) derivatives. First, the self-decomposition of PING· follows second-order kinetics. However, when excess of 4-Me-NHPI are used in the system, it was found that H-atom abstraction competes with the self-decomposition of 4-Me-PINO·. Second, the hydrogen atom self-exchange reactions between PINO· and substituted NHPI were found to follow H-atom transfer rather than the stepwise electron-proton transfer pathway. Last, the investigations of hydrogen abstraction from para-xylene and toluene by PINO· show large kinetic isotope effects, with the reaction becoming slower when the ring substituent on PINO· is more electron donating.